Subject: Re: [PATCH bpf-next 5/6] libbpf: add support for bpf_link-based cgroup attachment

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 23 Mar 2020 12:02:24 +0100

Andrii Nakryiko <[email-masked]> writes:

> Add bpf_program__attach_cgroup(), which uses BPF_LINK_CREATE subcommand to
> create an FD-based kernel bpf_link. Also add low-level bpf_link_create() API.
>
> If expected_attach_type is not specified explicitly with
> bpf_program__set_expected_attach_type(), libbpf will try to determine proper
> attach type from BPF program's section definition.
>
> Also add support for bpf_link's underlying BPF program replacement:
>   - unconditional through high-level bpf_link__update_program() API;
>   - cmpxchg-like with specifying expected current BPF program through
>     low-level bpf_link_update() API.
>
> Signed-off-by: Andrii Nakryiko <[email-masked]>
> ---
>  tools/include/uapi/linux/bpf.h | 12 +++++++++
>  tools/lib/bpf/bpf.c            | 34 +++++++++++++++++++++++++
>  tools/lib/bpf/bpf.h            | 19 ++++++++++++++
>  tools/lib/bpf/libbpf.c         | 46 ++++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h         |  8 +++++-
>  tools/lib/bpf/libbpf.map       |  4 +++
>  6 files changed, 122 insertions(+), 1 deletion(-)
>
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index fad9f79bb8f1..fa944093f9fc 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -112,6 +112,7 @@ enum bpf_cmd {
>  	BPF_MAP_UPDATE_BATCH,
>  	BPF_MAP_DELETE_BATCH,
>  	BPF_LINK_CREATE,
> +	BPF_LINK_UPDATE,
>  };
>  
>  enum bpf_map_type {
> @@ -574,6 +575,17 @@ union bpf_attr {
>  		__u32		target_fd;	/* object to attach to */
>  		__u32		attach_type;	/* attach type */
>  	} link_create;
> +
> +	struct { /* struct used by BPF_LINK_UPDATE command */
> +		__u32		link_fd;	/* link fd */
> +		/* new program fd to update link with */
> +		__u32		new_prog_fd;
> +		__u32		flags;		/* extra flags */
> +		/* expected link's program fd; is specified only if
> +		 * BPF_F_REPLACE flag is set in flags */
> +		__u32		old_prog_fd;
> +	} link_update;
> +
>  } __attribute__((aligned(8)));
>  
>  /* The description below is an attempt at providing documentation to eBPF
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index c6dafe563176..35c34fc81bd0 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -584,6 +584,40 @@ int bpf_prog_detach2(int prog_fd, int target_fd, enum bpf_attach_type type)
>  	return sys_bpf(BPF_PROG_DETACH, &attr, sizeof(attr));
>  }
>  
> +int bpf_link_create(int prog_fd, int target_fd,
> +		    enum bpf_attach_type attach_type,
> +		    const struct bpf_link_create_opts *opts)
> +{
> +	union bpf_attr attr;
> +
> +	if (!OPTS_VALID(opts, bpf_link_create_opts))
> +		return -EINVAL;
> +
> +	memset(&attr, 0, sizeof(attr));
> +	attr.link_create.prog_fd = prog_fd;
> +	attr.link_create.target_fd = target_fd;
> +	attr.link_create.attach_type = attach_type;
> +
> +	return sys_bpf(BPF_LINK_CREATE, &attr, sizeof(attr));
> +}
> +
> +int bpf_link_update(int link_fd, int new_prog_fd,
> +		    const struct bpf_link_update_opts *opts)
> +{
> +	union bpf_attr attr;
> +
> +	if (!OPTS_VALID(opts, bpf_link_update_opts))
> +		return -EINVAL;
> +
> +	memset(&attr, 0, sizeof(attr));
> +	attr.link_update.link_fd = link_fd;
> +	attr.link_update.new_prog_fd = new_prog_fd;
> +	attr.link_update.flags = OPTS_GET(opts, flags, 0);
> +	attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
> +
> +	return sys_bpf(BPF_LINK_UPDATE, &attr, sizeof(attr));
> +}
> +
>  int bpf_prog_query(int target_fd, enum bpf_attach_type type, __u32 query_flags,
>  		   __u32 *attach_flags, __u32 *prog_ids, __u32 *prog_cnt)
>  {
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index b976e77316cc..46d47afdd887 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -168,6 +168,25 @@ LIBBPF_API int bpf_prog_detach(int attachable_fd, enum bpf_attach_type type);
>  LIBBPF_API int bpf_prog_detach2(int prog_fd, int attachable_fd,
>  				enum bpf_attach_type type);
>  
> +struct bpf_link_create_opts {
> +	size_t sz; /* size of this struct for forward/backward compatibility */
> +};
> +#define bpf_link_create_opts__last_field sz
> +
> +LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
> +			       enum bpf_attach_type attach_type,
> +			       const struct bpf_link_create_opts *opts);
> +
> +struct bpf_link_update_opts {
> +	size_t sz; /* size of this struct for forward/backward compatibility */
> +	__u32 flags;	   /* extra flags */
> +	__u32 old_prog_fd; /* expected old program FD */
> +};
> +#define bpf_link_update_opts__last_field old_prog_fd
> +
> +LIBBPF_API int bpf_link_update(int link_fd, int new_prog_fd,
> +			       const struct bpf_link_update_opts *opts);
> +
>  struct bpf_prog_test_run_attr {
>  	int prog_fd;
>  	int repeat;
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 085e41f9b68e..8b23c70033d3 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6951,6 +6951,12 @@ struct bpf_link {
>  	bool disconnected;
>  };
>  
> +/* Replace link's underlying BPF program with the new one */
> +int bpf_link__update_program(struct bpf_link *link, struct bpf_program *prog)
> +{
> +	return bpf_link_update(bpf_link__fd(link), bpf_program__fd(prog), NULL);
> +}

I would expect bpf_link to keep track of the previous program and
automatically fill it in with this operation. I.e., it should be
possible to do something like:

link = bpf_link__open("/sys/fs/bpf/my_link");
prog = bpf_link__get_prog(link);
new_prog = enhance_prog(prog);
err = bpf_link__update_program(link, new_prog);

and have atomic replacement "just work". This obviously implies that
bpf_link__open() should use that BPF_LINK_QUERY operation I was
requesting in my comment to the previous patch :)

-Toke